To determine if the adjective form is correct, look at the gender of the subject and ascertain whether it is singular or plural. The correct adjective form will be the same gender as the noun and will also match the noun's singularity or plurality.
"Mi mamá es amable, atlético, y generoso."
The subject (mi mamá) is feminine, but two of the adjectives - atlético, generoso - are masculine, making the adjective form incorrect. To make them feminine, change the -o at the end to an -a.
"Mi mamá es amable, atlética, y generosa."
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"La casa es grande, roja, y bonita."
La casa is feminine and singular; grande, roja, and bonita are all feminine and singular. The adjective form is correct.
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"Los jóvenes son altos, fuerte, y rápido."
Los jóvenes - masculine, plural. Altos - masculine plural. Fuerte, rápido - masculine, singular. Add an s to make the adjectives plural!
"Los jóvenes son altos, fuertes, y rápidos."
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That's really the key to adjective form. I hope this helps you!